IMO it's because pixel programs are more similar to painting and drawing, and they also have more name recognition/market share. The majority of 2D artists are introduced to a pixel program first and probably use it for years before ever being exposed to a vector program. There are also some stylistic things that are very hard to do with vector. Both are hard to sprite with because most of the programs available include little or no animation support; the animation specialized ones tend to be crappy for producing the art in.

For what it's worth, raster graphics are pixels. I think you mean "why are they using pixels/raster graphics instead of vector graphics?"

I don't know if you're talking about using vector graphics for the creation of the game's art, or if you're talking about using vector graphics to render the game's art, which are two different things. Some games will create their assets using vector graphics, but then rasterize them and export pixels/bitmaps to be used when rendering in the game.

As far as I know, one of the main reasons vector graphics aren't used in the rendering stage is because that's not how your computer works. Your screen is just a large raster. It's easier (and faster) to take one raster (your pixelated bitmap sprite) and copy it to another raster (the screen) than it is to take a set of vector graphics (your vectorized sprites) and then rasterize them to the screen. OpenGL/D3D work with rasters and not vectors when it comes to textures. It's easier to work with the tools you have (OpenGL/D3D, which also give hardware acceleration) than it is to reinvent them (i.e. implement vector graphics rendering for a real-time game).

Actually, many 2D games use vector graphics (including pretty much most Flash games or games where the art was made using Flash).

But I'm going to add another point to the ones already mentioned: originally raster was pretty much the only way to go for a lot of time, and 2D games were made for that. So as you can imagine there's some inertia carrying from those times, with 2D artists having been mainly trained for raster graphics instead of vector graphics.

The biggest problem with bitmaps is that they don't scale up with larger resolutions (the "pixels" start getting large), and large bitmaps can easily take up lots of memory. If I was to make a 2D game, I'd go with bitmaps if the game only runs at a low resolution, and vector graphics if it can be run at higher resolutions.

Photoshop is hugely expensive, other than that people love it, as far as I've seen. Gimp and Paint Tool Sai are the two other commonly used pixel programs, from the amount I've heard other people discussing them. I'm a vector artist myself, I mainly use Inkscape because it's free software.

And can anyone recommend good pixel art tools? I currently use Graphics gale.

Personally when it comes to pixelart I have yet to find any tool that's better than GraphicsGale. I have tried several times but I couldn't, period (regardless of the license, even!). It seems almost all drawing programs focus in either Photoshop-style editing or vector graphics, and the few drawing tools made for pixelart I could find simply feel like a joke in comparison to GraphicsGale (especially when it comes to manipulating the palette).

Photoshop is awesome for general photo editing or rasterized drawing, however its main purpose is high-quality, high-resolution images. It's not really meant for pixelated* or 8/16bit sprites. I've never tried Graphics Gale, but I've never liked Photoshop or Gimp for making pixelated* or 8/16bit sprites.

*By pixelated I mean images with large pixels, not general raster graphics.